In heat pumps of the absorption type, an absorbent, diluted with an absorbed refrigerant, is heated in a generator to vaporize some of the refrigerant. The refrigerant vapor then flows to a condenser where it is condensed to a liquid by heat exchange with an external cooling fluid maintained at a low temperature by a heat sink. The liquefied refrigerant then flows through a valve to an evaporator which vaporizes the refrigerant (usually at low pressure) to produce refrigeration.
The vaporized refrigerant then flows to an absorber where it is absorbed by concentrated absorbent supplied from the generator. From the absorber, the diluted absorbent passes to the generator where it is concentrated by heating to vaporize some of the refrigerant, and thus repeat the cycle.
Conventional absorption heat pumps typically employ an aqueous solution of lithium bromide as an absorbent and water as a refrigerant. The operating efficiency of these heat pumps increases with the difference between the highest fluid temperature where the solution is dilute in lithium bromide and water is being vaporized, and the lowest fluid temperature where the solution is very concentrated in lithium bromide and water is being absorbed. When operating in a refrigeration/air conditioning mode, the high temperature is fixed by the ambient temperature. When operating in a temperature boosting mode, the high temperatures can reach 98.degree. C., 177.degree. C., and 232.degree. C. for single, double and triple effect machines, respectively. Some of these machines are described in Herold and Radermacher "Absorption Heat Pumps". Mechanical Engineering, August 1989, pp.68-73. Since the high cycle temperature is generally fixed by the application and/or pump type, the efficiency of the cycle can be increased by lowering the low cycle temperature.
As the low cycle temperature is reduced in an air conditioning application, the concentration of lithium bromide must be increased in order to permit the continued absorption of water vapor. As the salt concentration is increased and the temperature is decreased, a solubility limit is approached. If the solubility limit of lithium bromide in water is exceeded, hydrated salt crystals may form which block the flow circulation in the absorber, rendering it useless. Thus, conventional absorption heat pumps use solutions containing about 60-62% salt, and operate at a minimum fluid temperature of about 4-7.degree. C. in air conditioning applications. For heating applications, the salt concentration may be lowered, to prevent freezing of the solution at temperatures down to -25.degree. C. or lower.
Absorption heat pumps have many large-scale uses in industrial air-conditioning and refrigeration, as well as heating and temperature boosting. There is always a need or desire for more efficient heat pumps which maximize the difference between the high and low fluid temperatures at different parts of the cycle.